1. Field of the Invention
This invention relates generally to electrical interconnections between a ceramic substrate and a supporting circuit board and more particularly to a method of forming a ball grid array of conductors on the ceramic substrate and to a product formed thereby.
2. Related Art
In microelectronic applications (e.g., for electronic circuit packages which may include one or more integrated circuits), solder bonds are commonly used to attach the package to a substrate such as a printed circuit board. In one technique, an electronic package is connected to a printed circuit board, both electrically and thermally, by the use of multiple solder balls in an array. The package is placed in registration with the printed circuit board and heated until the solder balls of such an array flow and collapse to a limited degree to effect connection to terminals on the printed circuit board or other substrate.
The use of solder ball electrical connectors is shown in Steitz, U.S. Pat. No. 3,719,981, where solder balls are shown attached to low profile solder bumps to provide a workable configuration for connection to a printed circuit board. Steitz uses a tacky, pressure sensitive tape for maintaining an alignment of the solder balls until they are positioned into wells within a mask and onto the solder bumps and reflowed. However, the process in Steitz is lengthy, involving many steps, and does not provide adequate control over the ultimate size of the solder balls, which is critical to making reliable connections, particularly where there is a high density of solder balls.
Angulas, et al., U.S. Pat. No. 5,203,075, discloses formation of interconnections between circuit-carrying substrates by heating solder paste deposits, causing the solder to melt and ball up around a solder ball attached to a circuit-carrying substrate opposite the one on which the deposit of solder paste is located. Angulas, et al., however, requires the solder balls to be arranged in a template from which they are transferred to the circuit-carrying substrate.
Angulas, et al., U.S. Pat. No. 5,133,495, discloses a method of forming solder balls in an array on a circuit substrate by heating deposits of solder paste surrounded by organic dewetting material so that the solder paste forms balls electrically connected with conductors located beneath the deposits of solder paste. This method, however, requires very precise control over the deposition of the solder paste and an underlying deposit of anti-wetting material to produce an array of solder balls of uniform size.
Thus, in the past it has been difficult to construct an array of substantially similar-sized spherical balls of solder protruding from an area of an integrated circuit package for use in mounting and connecting such a package on a printed circuit or similar substrate. What is still needed, then, is an improved method for producing an array of solder balls attached to a substrate of an electronic circuit package so that the balls are of uniform height, as well as an electronic circuit package including such an array of solder balls closely spaced together, accurately located, and of uniform height, so that the solder balls can all be connected reliably to an array of circuit terminals.